1. Field of the Invention
This invention relates to a method and an apparatus for controlling feedwater flow to a steam generating device, and more particularly to a method and an apparatus for controlling feedwater flow to a nuclear reactor, which method and apparatus are suitable for preventing an excessive rise of the water level in the nuclear reactor in the event of a scram of the nuclear reactor.
2. Description of the Prior Art
Before a scram occurs in a nuclear reactor, steam bubbles of suitable amounts are present below the surface of water in the nuclear reactor, and the water level in the nuclear reactor is maintained substantially constant. However, in the event of a scram in the nuclear reactor, the source of heat supply in the nuclear reactor is quickly shut off, resulting in a quick decrease in the amounts of the steam bubbles, and this is followed by a great drop of the water level in the nuclear reactor. This variation in the water level is dependent upon the factors including the variation in the flow rate of main steam, the variation in the internal pressure of the nuclear reactor, and the variation in the flow rate of feedwater. Practical experiences have taught the fact that the water level in the nuclear reactor is not simply raised by the action of the valves including the main stop valves, the turbine control valves, and safety-relief valves but rises monotonously after repeating a rise and a fall a plurality of times.
It is desirable to maintain the water level in the nuclear reactor at its normal operation level even in the event of occurrence of such a transient phenomenon. However, once the water level exceeds the normal operation level, it is impossible to lower the water level again since the flow rate of main steam has been decreased. Further, suppression of the increase in the flow rate of feedwater for the purpose of preventing the undesirable excessive rise of the reactor water level in the event of the reactor scram results inevitably in various problems including an excessive drop of the reactor water level.
Various control methods have heretofore been employed to deal with such problems and one of such methods is disclosed in the Japanese laid-open patent publication No. 44793/78. According to one of the prior art control methods, the feedwater pump is tripped manually in a predetermined length of time after the nuclear reactor has scrammed.
According to another prior art control method, the set point of the water level is automatically lowered temporarily upon detection of the occurrence of the scram in the nuclear reactor. However, these prior art control methods have not been completely satisfactory, and stabilization of the water level at the desired normal operation level after its transient variation has been extremely difficult especially when the water level varies in a complex fashion.